1. Field of the Invention:
The invention relates to frequency selective limiters (FSLs) and in particular to an FSL having an improved limiting response over its bandwidth.
2. Description of the Prior Art
FIG. 1 illustrates a typical configuration of a known FSL 10. A planar YIG substrate 12 carries a conductor 14 in a known manner. The YIG 12 and conductor 14 are aligned along an axis 16. Biasing magnets 18 having opposed pole pieces 20 provide a magnetic field H. The conductor 14 is parallel to the poles 20 and lies perpendicular to the field H.
It should be understood that the field H may be rotated 90.degree. about the axis 16. This may be accomplished by repositionment of the magnets 18 and pole pieces 20 to the positions shown in phantom. In either case the magnetic field H is perpendicular to the conductor 14 along the axis 16.
Known FSLs have a frequency dependent response to incoming signals as illustrated by curve 22 in FIG. 2. The bandwidth is about 3 GHz with a low frequency at about 2.4 GHz and the high frequency at about 5.4 GHz. Although the actual values vary within the bandwidth, in general the attenuation versus frequency response of the upper two-thirds of the curve 22 is generally flat and the lower one-third rolls off at low frequencies. Thus, attenuation of the low frequency signals is reduced and it becomes more difficult to distinguish low power signals of interest in the presence of high power signals at the low frequency end of the bandwidth. Ideally the curve 22 should be flat across the bandwidth including the low frequency end as illustrated by the dotted line 24.
Low frequency roll off is undesirable because it leaves a zone of vulnerability. While it may be possible to increase low frequency sensitivity, such an increase is achieved at the cost of adding additional FSL stages to those already in use which increases the delay between the detection of a signal and the processing of a signal. When the delay time is increased to unacceptable levels, the system cannot respond quickly enough to process the incoming information and allow for appropriate countermeasures. Also, additional FSL stages undesirably add to the cost and weight of the FSL and reduce volume efficiency.